It seems that the same behavior is observed for v1.6.
Interestingly, I tried my code for medium-complicated functions and it scales reasonably. Currently, I am investigating the issue for the most complicated function (it is with such function when I encounter the problem discussed herein). If it turns out it is something related to my specific function, it is a problem I have to solve myself. Otherwise, I will reopen the discussion.
Apologies and thank you very much.
shce